The invention relates to a method and apparatus for the recovery of plastic materials from starting pieces of composite fiber materials including fiber fabrics preimpregnated with a matrix plastic material forming prepreg structures and covered by protective films. German Patent Document DE41 16 638 A1 relates to plastic materials of the type contemplated by the present invention.
In aircraft construction and increasingly also in automobile and sports equipment construction, particular demands are placed on the properties of the materials used, which can be fulfilled by using sometimes very expensive composite fiber materials, so-called prepregs (preimpregnated). These prepregs usually consist of carbon fiber fabrics which are preimpregnated with a matrix plastic material. The matrix plastic materials used are preferably strongly adhesive epoxy or phenolic resins. Further examples of such self-adhesive composite plastic materials are glass fiber fabrics which are likewise impregnated with phenolic resin. In order to enable the adhesive fabrics to be stored, transported and cut to size, the fabrics are provided on both sides with protective films of polyethylene or impregnated paper. During the processing of such prepregs, supplied in rolled webs, e.g. in the manufacture of rudder units for aircraft, non-cured cutting residues of these expensive prepregs occur. The cutting residues currently have to be disposed of as special waste, which is cost-intensive. Owing to the high value of the material and the cost intensive disposal of the cutting residues, there is an urgent requirement to find more economical possibilities of recovering and reusing these materials. In this regard, on the one hand preservation usually of large areas of the fabric structure, but especially preservation of the fiber structure is to be guaranteed, i.e. fiber damage is to be avoided and, on the other hand, a chemical modification of the reactive resin and thus a permanent change to the adhesive strength of the fabric are to be avoided. Hitherto there have only been manual methods for dividing and separating self-adhesive, flexible films from a flexible backing material. However, manual separation of the protective films from the prepreg residues is very expensive and makes a manually assisted recovery method economically inviable.
Dwindling reserves of raw materials and diminishing landfill areas for waste disposal have led to the development of various recycling methods. In this regard, the recycling of composite materials places particular demands on such methods. Firstly, the composite material has to be comminuted and, in a following step, the composite particles thus obtained have to be divided into their individual material components and segregated from any mixture.
Currently applied methods for cutting up or dividing the composite of different materials are based on the selective separation of individual composite components or of the adhesive agent [Mang, T,; Umwelt 24, p.152-153, 1994] or on the selective comminution of individual composite components, cf. for example the publication DE 42 16 638 A1 referred to above. In some cases, this selective comminution is only made possible by cooling down to the temperature of liquid nitrogen (-196.degree. C.) and thus causing the material to become brittle [Burmester, F.; Mull Abfall 16(6), p.183-186, 1984]. Such methods cannot be applied if the composite component(s) to be recycled must not be damaged.
In the subsequent separation of material mixtures, differences in the densities of the individual components of the mixture are currently usually exploited. Examples which can be mentioned here are the separation of plastics mixtures by means of dry methods, such as wind sifting [Melchiorre, M., Guldenpfennig, M., Lohr, K., Zurn, J.; Chemie-Ingenieur-Technik 66(5), p.661-670, (1994)] or by means of wet methods, such as float and sink separation, or by means of hydrocyclones [Bahr, A.; Erzmetall 33, p.324-330, (1980)]. In addition to the density dependence, the methods mentioned first and last also have developments of the method in order to be able to take account of influences of the particle shape and particle sizes in the selection. To apply the float and sink method, a homogeneous liquid of a suitable density must be available, e.g. a solution or a dispersion or emulsion. In the specific case applicable here of sorting plastics mixtures with adhesive and/or chemically reactive surfaces, the potential for choice of the liquid separating medium is greatly restricted. The dry methods mentioned cannot be applied for sorting strongly adhesive materials. Since the cutting material to be treated in this case occurs in different shapes and sizes and usually above the maximum particle size suitable for hydrocyclones, hydrocyclones cannot be used.
Another method for separating material mixtures is flotation [cf. for example EP 535 419 A1]. Instead of density, this method uses the wettability of the particle surface as the selection parameter. This selection parameter is exploited in the flotation method by suspending the comminuted mixture in a liquid (usually water) and there subjecting it to a stream of rising gas bubbles (usually air). These gas bubbles accumulate selectively on the mixture components in accordance with the wettability thereof. Good wettability of the particle surface by the suspension liquid brings about poor bubble accumulation and vice versa. Mixture components which have a sufficient accumulation of bubbles rise to the surface; the other components remain in the suspension. Mixtures actually occurring do not usually have sufficient differences in the wettability of their components to enable sorting to be carried out by means of flotation. The wettability of the mixture components must therefore be selectively modified by so-called conditioning, i.e. by adding surface-reactive chemicals to the suspension. In the present specific case of sorting plastics mixtures with adhesive and/or chemically surfaces, the potential for choice of such conditioning reagents is greatly restricted.
An object of the invention is to divide such self-adhesive composite materials, such as are represented by the abovementioned film-lined cutting residues of prepreg webs or similar self-adhesive webs, automatically into the individual substance components, taking into account the restrictions mentioned, and to isolate, i.e. to separate, the materials from the mixture thus produced.
According to the invention, this object is achieved by providing a method comprising the following sequential steps:
mechanically comminuting the starting pieces into shreds of composite material; PA1 cooling the shreds of composite material to temperatures below ambient room temperature; PA1 subjecting the shreds of composite material to alternating mechanical stresses and thereby scaling off the protective films from the prepreg structures to form a mixture of film clippings and free prepreg pieces; PA1 and subjecting the mixture of film clippings and free prepreg pieces to a wet density selective separation process.
According to preferred embodiments of the invention, the adhesive strength of the resin is firstly weakened in a purely physical manner by cooling, preferably in an aqueous medium. Subsequently, alternating bending, axial and/or shearing stresses divide the weakened composite material into its components as a result of mechanical deformations of the pieces of fabric so that the film clippings become detached from the prepreg pieces in a scale-like manner. An aqueous medium reduces the adhesive effect of the resin sufficiently to prevent the pieces of fabric adhering to one another or to the protective films or to the walls of the processing plant. The divided components (prepreg fabrics and protective films) have sufficient differences in their densities, at least after an accumulation of air, to make density separation possible. Thus, for example, strongly adhesive carbon fiber fabrics impregnated with epoxy resin or even glass fiber fabrics impregnated with phenolic resin can, by means of this method, be divided and separated from the protective films surrounding them made of polyethylene or even of impregnated paper. It is also conceivable, in the case of web pieces and lining films of equal density, instead of the density-selective float and sink method, even to envisage a pure, i.e. wetting-selective, flotation method with preceding selective conditioning of the components. In this case, it may be necessary to pay careful attention in the selection of the conditioning agents to the fact that they do not impair the reactivity of the web materials to be recovered.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.